namespace nbody
{
	/*  The Computer Language Benchmarks Game
		https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
		contributed by Isaac Gouy
		modified by Robert F. Tobler
		modified by Eric P. Nusbaum
	*/

	using System;

	public class NBody
	{
		public static void Main(String[] args)
		{
			int n = args.Length > 0 ? Int32.Parse(args[0]) : 10000;
			NBodySystem bodies = new NBodySystem();
			Console.WriteLine("{0:f9}", bodies.Energy());
			for (int i = 0; i < n; i++) bodies.Advance(0.01);
			Console.WriteLine("{0:f9}", bodies.Energy());
		}
	}

	public class Body { public double x, y, z, vx, vy, vz, mass; }

	public class NBodySystem
	{
		private Body[] _bodies;
		private Body[] _pairL;
		private Body[] _pairR;
		private byte bodyCount = 5;

		const double Pi = 3.141592653589793;
		const double Solarmass = 4 * Pi * Pi;
		const double DaysPeryear = 365.24;

		public NBodySystem()
		{
			_bodies = new[]
			{
				new Body()
				{
					// Sun
					mass = Solarmass,
				},
				new Body()
				{
					// Jupiter
					x = 4.84143144246472090e+00,
					y = -1.16032004402742839e+00,
					z = -1.03622044471123109e-01,
					vx = 1.66007664274403694e-03*DaysPeryear,
					vy = 7.69901118419740425e-03*DaysPeryear,
					vz = -6.90460016972063023e-05*DaysPeryear,
					mass = 9.54791938424326609e-04*Solarmass,
				},
				new Body()
				{
					// Saturn
					x = 8.34336671824457987e+00,
					y = 4.12479856412430479e+00,
					z = -4.03523417114321381e-01,
					vx = -2.76742510726862411e-03*DaysPeryear,
					vy = 4.99852801234917238e-03*DaysPeryear,
					vz = 2.30417297573763929e-05*DaysPeryear,
					mass = 2.85885980666130812e-04*Solarmass,
				},
				new Body()
				{
					// Uranus
					x = 1.28943695621391310e+01,
					y = -1.51111514016986312e+01,
					z = -2.23307578892655734e-01,
					vx = 2.96460137564761618e-03*DaysPeryear,
					vy = 2.37847173959480950e-03*DaysPeryear,
					vz = -2.96589568540237556e-05*DaysPeryear,
					mass = 4.36624404335156298e-05*Solarmass,
				},
				new Body()
				{
					// Neptune
					x = 1.53796971148509165e+01,
					y = -2.59193146099879641e+01,
					z = 1.79258772950371181e-01,
					vx = 2.68067772490389322e-03*DaysPeryear,
					vy = 1.62824170038242295e-03*DaysPeryear,
					vz = -9.51592254519715870e-05*DaysPeryear,
					mass = 5.15138902046611451e-05*Solarmass,
				},
			};

			_pairL = new Body[(bodyCount * (bodyCount - 1) / 2)];
			_pairR = new Body[(bodyCount * (bodyCount - 1) / 2)];
			var pi = 0;
			for (var i = 0; i < bodyCount - 1; i++)
				for (var j = i + 1; j < bodyCount; j++)
				{
					_pairL[pi] = _bodies[i];
					_pairR[pi] = _bodies[j];
					pi++;
				}

			double px = 0.0, py = 0.0, pz = 0.0;
			foreach (var b in _bodies)
			{
				px += b.vx * b.mass; py += b.vy * b.mass; pz += b.vz * b.mass;
			}
			var sol = _bodies[0];
			sol.vx = -px / Solarmass; sol.vy = -py / Solarmass; sol.vz = -pz / Solarmass;
		}

		public void Advance(double dt)
		{
			var length = _pairL.Length;
			for (int i = 0; i < length; i++)
			{
				Body bi =  _pairL[i], bj = _pairR[i];
				double dx = bi.x - bj.x, dy = bi.y - bj.y, dz = bi.z - bj.z;
				double d2 = dx * dx + dy * dy + dz * dz;
				double mag = dt / (d2 * Math.Sqrt(d2));
				bi.vx -= dx * bj.mass * mag; bj.vx += dx * bi.mass * mag;
				bi.vy -= dy * bj.mass * mag; bj.vy += dy * bi.mass * mag;
				bi.vz -= dz * bj.mass * mag; bj.vz += dz * bi.mass * mag;
			}
			foreach (var b in _bodies)
			{
				b.x += dt * b.vx; b.y += dt * b.vy; b.z += dt * b.vz;
			}
		}

		public double Energy()
		{
			double e = 0.0;
			for (int i = 0; i < bodyCount; i++)
			{
				var bi = _bodies[i];
				e += 0.5 * bi.mass * (bi.vx * bi.vx + bi.vy * bi.vy + bi.vz * bi.vz);
				for (int j = i + 1; j < bodyCount; j++)
				{
					var bj = _bodies[j];
					double dx = bi.x - bj.x, dy = bi.y - bj.y, dz = bi.z - bj.z;
					e -= (bi.mass * bj.mass) / Math.Sqrt(dx * dx + dy * dy + dz * dz);
				}
			}
			return e;
		}
	}
}
